Historically, the efficiencies and lifetimes of organic electroluminescence (“EL”) devices were much lower than those obtained from inorganic systems. Therefore, research mainly focused on inorganic materials.
The reason for the low luminance of the early organic EL device is the highly resistive EL medium, which prevents the efficient injection of carriers into the light-emitting layer. Tang and VanSlyke addressed this problem in the late 1980s (Tang and VanSlyke, Appl. Phys. Lett. 1987, 51, 913) by using a structure made of two ultra thin layers: a hole transporting layer of an organic substance laminated on an organic emitting layer. This work revived the research on organic EL devices, and resulted in the development of a new generation of light-emitting diodes with organic dyes. One of the most convenient and useful methods is to dope a strong emitting material into a host material to form a guest-host system. Thus, in principle, an organic EL device with good efficiency and high stability, as well as desired color with proper chromaticity, can be obtained by doping different strongly emitting materials into a host material such as tri-(8-hydroxyquinolinato)aluminum (Alq3) to meet the requirement of the practical applications. As a general rule, the energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of a host material should be larger than that of the doped guest material to allow an efficient energy transfer from the host to guest.
Light emission from OLEDs is typically via fluorescence or phosphorescence. For OLEDs based on fluorescent materials, only a small percentage (about 25%) of excitons in fluorescent devices are capable of producing the fluorescent luminescence that is obtained from a singlet-excited state. The remaining excitons in a fluorescent device, which are produced in the lowest triplet excited state of an organic molecule, are typically not capable of being converted into the energetically unfavorable higher singlet excited states from which the fluorescence is produced. This energy would be lost via non-radiative decay processes that only tend to heat-up the device. An efficient way to tape the other 75% excitons for light emission is to use materials in which the triplet excited state can decay to the ground state efficiently in radiative way, i.e. phosphorescence. The advantage is that all excitons, either in singlet or triplet excited states, may participate in luminescence. This is because the lowest singlet excited state of an organic molecule is typically at a slightly higher energy than the lowest triplet excited state. This means that, for typical phosphorescent organometallic compounds, the lowest singlet excited state may rapidly decay to the lowest triplet excited state from which the phosphorescence is produced. For this reason, there is much interest in finding efficient electrophosphorescent materials and OLED structures containing such materials, especially those emitting in saturated red.
It would therefore be desirable to provide an improved or alternative compound for use in organic light emitting devices that overcomes the problems associated with the prior art.